Artificial white light becomes eye-friendly

Tired eyes and the impression that white is artificial are known to everyone who spends time in places lighted by popular non-thermal sources like fluorescent lamps or LEDs. Scientists from laboratories all over the world have been trying to eliminate these unpleasant side effects for years in their search for methods to recreate sunlight, which is the most natural light for humans. In the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS), it was shown that this objective could be achieved. "We have discovered a class of organic molecules emitting white light with continuous spectrum covering almost the entire visible range," says Dr Jerzy Karpiuk from the IPC PAS who heads the research team. It is also important that the emission of the white light was obtained from one chemical compound with a very simple structure.

White is a special colour which is created as a result of mixing light of all wavelengths in the visual range -- i.e., from approximately 420 to 730 nanometres (one nanometer is one billionth part of a meter). The white colour of fluorescent lamps and similar artificial sources is created by the mixture of three colours only: red, green and blue, which come mainly from the non-continuous emissions of various inorganic (halophosphate or triphosphate) luminophores. The light obtained in this way is devoid of many colour components, and it is this effect that is responsible for the unpleasant visual sensations. In addition, the need to use several substances lowers the energetic efficiency of light sources and complicates their manufacturing technology.

The research team composed of Jerzy Karpiuk (IPC PAS), Ewelina Karolak (IPC PAS) and Jacek Nowacki (Faculty of Chemistry of the University of Warsaw) observed white light emission continuously covering virtually the entire visible range. Its source is crystal violet lactone (CVL), a substance produced in mass quantities and commonly used in copy paper as the so-called dye precursor. A CVL molecule has two fluorophores embedded in its structure and responsible for the emission of light: one for blue and the other one for orange. The contribution of each of them to CVL's dual fluorescence heavily depends on the environment of the molecule which modifies the energetics of their excited states. "By properly adjusting the molecule's surrounding, it is possible to control the parameters of the emission spectrum, and consequently, to change the colour or shade of the white light obtained," says Ewelina Karolak, a PhD student from the IPC PAS.

"The deeper significance of our research lies in the discovery that white fluorescence is a general property of CVL type molecules. The dependence of excited state energetics on molecular structure allows to predict the width, shape and other parameters of the dual fluorescence spectrum, and so enables the engineering and customised design of white fluorophores," adds Dr Karpiuk. It turns out that even very small molecules can emit continuous white light. This fact opens up a new perspective for the construction of innovative eye-friendly light sources.

The emission of white light by molecular structures as simple as CVL is highly desired and wanted phenomenon, mainly because of its potential use in organic light-emitting diodes. However, it is still a long way before it can be used in practice because crystal violate lactone emits light of low intensity and CVL-based light sources would not be efficient enough to be manufactured on a mass scale. "However, the most important thing is that we managed to show that a certain concept works in practice. Now we are sure that it is only a matter of time before light sources recreating natural white light will be constructed," Dr Karpiuk sums up the discovery.

An article on the discovery of white fluorescence was published in Physical Chemistry Chemical Physics journal.